Microelectromechanical Systems (MEMS), which are sometimes called micromechanical devices or micromachines, are three-dimensional objects having one or more dimensions ranging from microns to millimeters in size. MEMS have been developed with scanning mirrors, referred to as scanning micromirrors. Such scanning micromirrors can be used in a variety of applications including barcode readers, laser printers, confocal microscopes, and fiber-optic network components such as optical switch arrays. The devices are generally fabricated utilizing semiconductor processing techniques, such as lithographic technologies. Electrostatic combdrive actuators have been developed to actuate such scanning micromirrors. Unfortunately, the combdrive actuator portion of a micromirror device can take up a large portion of chip space. The space taken up by the combdrive actuator can be a limiting factor in the number of micromirror devices that can be placed on a chip. Packing the micromirror devices closer together is desirable, e.g., in optical switch arrays, because it reduces optical path length and allows for smaller mirror design. The current state of the art of combdrive actuators is described, for example, in U.S. patent application Ser. No. 09/584,835 entitled “Staggered Torsional Electrostatic Combdrive and Method of Forming Same” to Robert A. Conant and Jocelyn T. Nee, Kam-Yin Lau and Richard S. Muller, which was filed May 31, 2001.
FIG. 1 illustrates a Staggered Torsional Electrostatic Combdrive (STEC) 20 of the prior art. The STEC 20 includes a stationary combteeth assembly 22 and a moving combteeth assembly 30. The stationary combteeth assembly has individual combteeth 24 formed on a spine 26. The moving combteeth assembly 30 includes individual combteeth 32 linked by a spine 34. The moving combteeth assembly 30 also includes a mirror or paddle 40 with associated torsional hinges 42. In a resting state a moving combteeth assembly 30 is positioned entirely above the stationary combteeth assembly 22 as shown in FIG. 1. A typical prior art process flow involves creating the moving combteeth assembly 30 and the mirror 40 out of the same device layer of a silicon-on-insulator (SOI) wafer.
FIG. 2 illustrates the STEC system 20 in an activated state. This state is achieved by applying a voltage between the moving combteeth assembly 30 and the stationary combteeth assembly 22. In this state, the individual combteeth of the two assemblies interdigitate. The applied voltage attracts the moving combteeth assembly 30 to the fixed combteeth assembly 22, thus exerting torque on the torsional hinges 42, forcing the mirror 40 to tilt. The torsional hinges 42, which are anchored, provide restoring torque when the voltage is removed. Note that the combteeth assemblies 22, 30 take up space that might otherwise be used for additional mirrors. This restricts the density to which devices such as the STEC 20 can be packed. The packing density might be improved somewhat by forming the combteeth 32 at the edges of the mirror 40, but the combteeth 32 would still take up space and restrict the packing density of the mirrors.
Thus, there is a need in the art, for a combdrive device that can be densely packed and a method for fabricating it.